[Discussion] can we stop arguing about reconstitution until somebody posts a number
can we stop arguing about reconstitution until somebody posts a number — that is what I am asking, and I have already read the wiki twice.
The failure modes I have actually seen posted here, in order of frequency.
One: reconstituting at a different volume than intended and not writing it down, so the next draw is based on a remembered concentration. Two: treating units as a dose rather than a volume, which breaks the moment the concentration changes. Three: assuming a syringe is U-100 when it is not.
All three are solved by the same two habits — do the arithmetic before the water goes in, and write the result on the glass. Neither costs anything and between them they cover almost every question this board gets.
The arithmetic in full: concentration is mass over volume, so 15mg into 1mL gives 15mg/mL. A U-100 syringe is graduated at 100 units per millilitre, so 1.5mg is 1.5 divided by 15 millilitres, which is 10 units.
Started reconstituting everything to a round concentration and the six-in-the-morning error rate went to zero.
Corrections welcome, especially the pedantic ones. Pedantry is how this board earns its reputation.
best — the order this archive was captured in
Adding more water does not change how much peptide is in the vial. It changes the concentration, which changes the volume you draw for the same amount. That is the entire relationship.
The lyophilised cake displaces a small volume, so the true concentration is very slightly lower than the arithmetic suggests. It is a fraction of a percent and it is dwarfed by fill tolerance.
Shook the first vial because nobody told me not to. It was fine, but I have swirled every one since.
Bacteriostatic water contains benzyl alcohol as a preservative, which is what makes repeated entry into the same vial reasonable. Plain sterile water has no preservative.
twice as much water halves the concentration and doubles the units for the same dose
Working has to be shown on arithmetic posts. Added a note asking for it rather than removing this one.
Working has to be shown on arithmetic posts.
units_not_mg has the units point exactly right. They are a volume on the barrel and nothing else.
Aim the stream down the wall of the vial rather than into the cake. Reconstitution is a solubility problem, not a mixing problem, and mechanical shear does nothing helpful.
Correction: that gives 25 units, not what you have written. 20mg into 2mL is 10mg/mL and the division follows from there.
if the maths gives you a fraction of a unit, change the concentration
Is that a U-100 syringe? The barrel markings change the answer completely.
This. Showing the working is the fastest way for somebody else to spot the mistake.
Did the maths give you a whole number of units, or a fraction?
Spent 19 weeks confused about this until somebody drew the table for me. 5mg, 1mL, 5mg/mL, 5mg is 100 units. That is the whole thing.
What is written on the vial right now?
Reconstituted at 3mL when I meant to use a different volume. Caught it because the units looked wrong on the barrel. Now I write 6.67mg/mL on the glass before I put the cap back.
the number you write down is the number you will trust at 6am
Show the arithmetic — where did the unit figure come from?
Asked SSA what volume they reconstitute to for their own stability testing and they answered in one line. Useful thing to know.
Are you accounting for the powder volume, or is that where the confusion is?
Are you accounting for the powder volume, or is that where the confusion is?
Right, and the arithmetic behind it: 20mg in 3mL is 6.67mg/mL, so 5mg is 75 units.
Careful — that arithmetic is wrong. 15mg in 2mL is 7.5mg/mL, so 0.75mg draws to 10 units, not what you wrote.
That is sterile water, not bacteriostatic. Same appearance, different product, and it matters for a multi-draw vial.
Small fix — a U-100 syringe is 100 units per mL. If yours is marked differently it is not U-100 and the whole calculation changes.
Not quite. Bacteriostatic and sterile water are not interchangeable for a multi-draw vial, and the difference is the preservative.
How many mg in the vial and how many mL are you adding?
label the vial with the date as well as the concentration
aim the stream at the glass wall, not at the cake
- 1Working has to be shown on arithmetic posts. Added a note asking for it…13 comments in this branch · started by u/units_not_mg
- 2Careful — that arithmetic is wrong. 15mg in 2mL is 7.5mg/mL, so 0.75mg draws…7 comments in this branch · started by u/janek_ferrari